Nanoscale laser harnesses plasmons

PORTLAND, Ore.  What is being billed as the world's smallest semiconductor laser boasts a 5-nm active region that is 20 times smaller than the wavelength of the light it emits, according to university researchers who recently demonstrated their nanoscale laser.

The key to scaling down the laser to the nanometer level was harnessing the interaction between light and surface plasmon waves, the researchers said.

Last month, experimental laser collaborators at Cornell University, Norfolk State University and Purdue University performed an alternative nanoscale laser demonstration, but their device was not integrated onto a chip.

"Ours is the smallest laser that can be integrated with semiconducting devices," claimed Guy Bartal, manager of the University of California at Berkeley's Nanoscale Science and Engineering Center. "There's another claim for a small laser that was published one week before us, but this one does not involve a technology that can be integrated with semiconductors."

Both pictures show a bright emerald green point of light from a single plasmon laser emanating from the optical set up used by researchers. (Courtesy of Xiang Zhang Lab, University of California at Berkeley)

Discrete semiconductor lasers used in telecommunications equipment are measured in millimeters, but micron-sized semiconductor lasers are currently under development in many labs. The two lasers recently demonstrated are the first to reach the nanometer realm.

While the UC-Berkeley researchers confined light to an area 20 times smaller than its wavelength, the Cornell-Norfolk State-Purdue laser confined light to an area 10 times smaller than its wavelength.

Reaching the nanometer level was achieved by harnessing the interaction between light and surface plasmon waves. Plasmons are quasiparticles that result from the quantization of electron oscillations similar to the way photons are quantizations of light waves.

Previously, optical resonators for semiconductor lasers had to be at least half the size of the laser light's wavelength. Using plasmons, the Berkeley researchers succeeded in confining the laser to a cavity much smaller than the light's wavelength.

The researchers were able to fabricate their nanoscale laser on a chip by depositing a cadmium sulfide nanowire atop an insulating gap measuring 5 nanometers. By pumping the nanowire with an external laser, they were able to achieve lasing in the transparent insulator below the nanowire.

They achieved "the hybridization of plasmons with a dielectric waveguide," said Bartal. "We found that by doing so, by incorporating a non-dielectric semiconducting nanowire on top of a metal, but separated from it by a really small gap, you can confine light to very small dimensions close to this five nanometers, but with much less dissipation."

Next, the researchers plan to demonstrate that their semiconductor structure can also lase when electrically stimulated, eliminating the need for an external laser to act as a pump. After that, the team plans to scale the laser into even smaller cavities. The ultimate goal is reaching the angstrom level, thereby enabling lasers to be constructed on the same scale as electrons. That approach also would ease the integration of hybrid devices that combine optics and electronics.

Rupert Oulton, a research associate at the UC-Berkeley lab, first theorized the nanoscale laser last year.

The nanoscale laser research was funded by the National Science Foundation and the U.S. Air Force Office of Scientific Research.